Special adjoint for broadcasted literal pow

Currently taking the gradient of anything that contains a broadcasted
literal pow adds RefValue{typeof(^)}(^) and a similar entry for the
literal power itself to the IdDict. This is probably because of the
special signature in the broadcasting machinery:

```
Base.broadcasted(Base.literal_pow, Main.:^, vec, Val{N}())
```

By adding a special adjoint for broadcasting literal_pow, not only
do we reduce the noise in the param's IdDict, but it also speeds
up taking the gradient of basic loss functions like sum(err.^2).

src/lib/broadcast.jl

@@ -76,6 +76,11 @@ Numeric{T<:Number} = Union{T,AbstractArray{<:T}}
   res, Δ -> (nothing, unbroadcast(x, Δ ./ y), unbroadcast(y, -Δ .* res ./ y))
 end
 
+@adjoint function broadcasted(::typeof(Base.literal_pow), ::typeof(^), x::Numeric, exp::Val{p}) where p
+  y = Base.literal_pow.(^, x, exp)
+  y, ȳ -> (nothing, nothing, ȳ .* p .* conj.(x .^ (p - 1)), nothing)
+end
+
 @adjoint broadcasted(::typeof(identity), x::Numeric) = x, Δ -> (nothing, Δ)
 
 @adjoint function broadcasted(::typeof(σ), x::Numeric)